In present-day communication systems communication links, in particular real-time connections, e.g., for voice, video or multimedia communication, are increasingly also routed via packet-oriented networks such as Local Area Networks (LANs) or Wide Area Networks (WANs). For example, Internet telephony, frequently also referred to as Voice over Internet Protocol (VoIP) telephony, is based on this technology.
These types of packet-oriented communication networks are frequently linked via gateway equipment to conventional switched-circuit communication networks, such as public or private Integrated Services Digital Network (ISDN) networks in order to implement connections across networks. Such a gateway facility between a packet-oriented and a switched-circuit communication network enables communication links to be routed from the switched-circuit into the packet-oriented communication network and vice versa. In this case the gateway facility performs the required signaling and protocol conversions between the signaling that are used in the packet-oriented and in the switched-circuit communication network.
Currently packet-oriented communication networks for setting up real time communication links are frequently based on ITU-T Recommendation H.323 or on the IETF Standard SIP (Session Initiation Protocol). This type of communication system is frequently also referred to as a VoIP system. The expression VoIP in this document is taken to refer to not only Internet protocol-based speech transmission but also to Internet protocol-based transmission of different media types such as video, fax, data and/or multimedia.
In a VoIP system the terminals and gateway facility belonging to the system register with a central signaling controller of the VoIP system in order to make themselves known in this way to the VoIP system. In VoIP systems in accordance with the H.323 Recommendation a signaling controller of this type is known as a gatekeeper. As well as the registration of terminals and gateway devices, another of the gatekeeper's functions is to resolve called destination addresses into Internet protocol-based transport addresses.
A VoIP system which is administered by one or more logically linked gatekeepers is frequently also referred to as a VoIP domain.
In accordance with the H.323 Recommendation a gateway device is handled by a gateway like a VoIP end point and, like other VoIP end points is assigned to precisely one gatekeeper by registration. On registration the gateway device stores its transport address with the gatekeeper assigned to it so that the gatekeeper can if necessary forward connection signaling to the gateway device.
Because of the assignment of a gateway device to precisely one gatekeeper a number of gateway devices are needed to link a number of VoIP domains to a switched-circuit communication network. However as a rule this requires a significant investment in hardware.